The concept of transmitting several television channels through optical fiber using analog intensity modulation of the output of a semiconductor laser diode has recently been receiving considerable attention. As proposed in the prior art, this would involve transmission of multi-channel amplitude modulated-vestigial side band (AM-VSB) signals, as used in present day cable television (CATV) systems, in an optical fiber transmission medium. Such an arrangement would be useful in a CATV trunk system or in a fiber-to-the-home network. Optical fiber transmission systems that use frequency division multiplexing overcome compatibility problems and have advantages such as simplicity of design, reduced bandwidth requirements for lightwave components, and much lower costs, as compared with optical time division multiplex (TDM) systems. For a discussion of such systems see, for instance, U.S. Pat. No. 5,111,475, incorporated herein by reference.
In order to achieve acceptable system performance, the lasers used in such a system must have a light output whose intensity is a very nearly linear function of the laser drive current under large-signal modulation. Strict limitations on laser nonlinearity are required because of the wide dynamic range of the National Television Systems Committee (NTSC) standard video format. Exemplarily, in the NTSC standard video format, the ratio of the magnitude of the carrier to the magnitude of the total third order intermodulation distortion products at the carrier frequency must be less than -65 dBc. Similarly, the peak second-order distortion, i.e., the sum of several tens of two-tone products (or the ratio of the carrier to the largest composite second-order peak), must be less than -60 dBc. To obtain such high signal quality in view of the large number of distortion products, the transmitting laser light-versus-current characteristic must be extremely linear. However, lasers that meet these linearity criteria are difficult to manufacture and typically have low manufacturing yield. Lasers for multichannel analog systems typically are distributed feedback (DFB) lasers, i.e., lasers that comprise a "diffraction grating".
The above cited '475 patent discloses some design features which can increase the yield of acceptable DFB lasers. However, although lasers according to the '475 patent can be manufactured at higher yield than some prior lasers, the yield is typically still substantially below 100%. This of course is undesirable since it results in relatively high unit cost of acceptable lasers. A laser design that can result in increased yield of low distortion lasers thus would have substantial economic significance. This application discloses a distributed feedback laser having a novel design feature that can result in increased yield of lasers acceptable for use in multichannel analog communication systems.